1. Field of the Invention
The invention relates to a process for transmitting a time signal from a satellite as well as process for receiving a time signal at a ground based receiver.
2. Description of the Related Art
A terrestrial time signal transmitter for example the DCF-77 transmitter of the Federal Institute of Physical Engineering at Frankfurt um Main, transmits its time signal in the long wave frequency band in order to facilitate long range transmission. However, despite the high transmitting power, a range of only 1,200 to 2,000 kilometres results. In addition this time signal is designed only for one national time and furthermore uses a special transmitter frequency) and intrinsic encoding, so that in an area in foreign countries the receiver has to be suitable for several different time signals or else it is no longer capable of reading the signal. In an area at sea far removed from the coast, reception is in general no longer possible. Setting the time with the aid of satellite positioning systems (GPS) is certainly possible, however in this country they lack the supplementary information such as daylight saving time, leap second and so on, so that an involved semi-manual adjustment is necessary in order to maintain the actual local time.
So-called multiple radio clocks are also known, which make it possible to receive or exploit time signals in different countries. However, it is necessary in this connection for the clock to be adjusted by the user so that the time of the place in which it is located is known. These multiple radio clocks are, however, not able to function in all countries.
A process for determining a position of a receiver is known from U.S. Pat. No. 5,408,444. In order to be able to set the correct time in this receiver, its position has to be determined using at least three satellites of the GPS satellite system. If the position is established, the time adjustment is carried out using a correction value for this position, the said correction value being filed in a data bank of the receiver.
Likewise from U.S. Pat. No. 5,574,670, it is known how to fix the position, by means of the GPS system, of a receiver situated on the ground. In addition, provision is made in the receiver situated on the ground to redirect the antenna to the respective orbit.
From DF 43 13 945 A1, several satellites are combined together to form a satellite system. For the position determination of the receiver, which is to receive the time signal, merely the doppler curve over time is used. However, position determinations of this type are very imprecise.
The purpose of the invention is to specify a process for transmitting and receiving a time signal, in which process simply a transmitter for determining the position of the receiver has to be provided in order to be able to set the actual local time.
The process of the invention provides that, to achieve global reception of the time signal, the transmission occurs from an aerospace vehicle moving relative to a point on the earth""s surface. The time signal is transmitted according to the invention with a particular frequency or several frequencies by the aerospace vehicle, which moves around an orbit with a high inclination. Further, provision is made that the signal rotates in the form of a beam at the transmitter in a predeterminable orbit and the rotating transmission beam contains angular information which is used to determine the direction of the transmitter. If the time signal transmits with one frequency, the distance between transmitter and receiver can be determined using the doppler curve. If several frequencies are used, the distance between transmitter and receiver can be determined by the propagation time scatter. In addition, because the angular information is acquired by the receiver from the rotating beam, the position of the receiver is determined in order to be capable of ascertaining whether the receiver is the left or right of a ground track of the aerospace vehicle. Consequently, an accurate position fixing of the receiver is possible, so that the actual local time can be set in the receiver. For determining the position of the receiver the radiated signal is therefore not radiated downwards uniformly, but rotates by means of a rotating beam. This rotation can be produced either by mechanically-driven antennas or by suitable electronic means. The rotating beam is altered in a suitable fashion as a function of the radiated angular position so that the instantaneous radiation angle can be determined from the received signal. This can be carried out for example by an auxiliary frequency, so that each angular position, or that is to say each range of angular positions between 0xc2x0 and 360xc2x0, has a defined auxiliary frequency. An angle of 90xc2x0 or 270xc2x0 at the time of the greatest convergence then defines the side of the flyby.
A receiver independently determines its own geographic position on the earth from the received signals of the time signal transmitter and fixes the actual local time from that, without user intervention being necessary. A normal satellite cannot be considered for such a time signal because either the altitude is too high because of the required life and consequently the required incoming-signal levels are not obtained or the inclination of the orbit is too low, so that the entire surface of the earth cannot be radiated. With a low-flying satellite or space station (at an altitude of for example 200 km to 400 km) with a high orbit inclination, it is possible however to cover the earth""s surface within the region of xc2x170 to 80 degrees of latitude. With a high orbit inclination, the entire earth""s surface is overflown in the course of time by the satellite or space station.
By means of a special antenna geometry of the device according to the invention, the scanned area of the earth""s surface can be expanded in width so that only the polar regions cannot be provided for
Terrestrial radio clocks are normally synchronised only once a day, in order to save the battery. This normally takes place at night because the changeover between daylight saving time and winter time also occurs at that time. With a space-supported radio clock, this is not so readily feasible since the transmitter must stay in the reception area for the given time. That is why the time signal transmitter transmits other supplementary data on the time of the next overflight for a particular area in addition to the basic time information, so that the receiver already knows in advance the contact time of reception. On first switching-on the clock or on losing the contact times the receiver switches on again only briefly in order to ascertain whether the time signal can be received. A quiet period is then inserted which is shorter than one reception time window, so that a possible contact cannot be missed. As soon as the first reception contact has been established, the clock goes over to the normal switching-on cycle.
The reception area for a particular point on the ground of the transmitter can extend over several time zones. That is why the receiver must determine, how far the instantaneous point on the ground, for which the transmitted data was calculated, is removed from its own geographic position. Two alternatives are provided for this:
1. During an overflight by the satellite on space station relatively close to the receiver, the doppler shift in the received frequency caused by the high velocity of the transmitter is so large that the time of the overflight, and therefore the distance, can be determined from the sudden change in frequency and from the form of the frequency jump.
2. During a relatively far distant flyby of the transmitter, the propagation time scatter of different frequencies (and therefore the dependency of the wave motion velocity of propagation on the wavelength or frequency) while passing through the earth""s ionosphere is exploited. The electrically-conducting upper atmospheric layers (ionosphere) impede the propagation radio waves depending on the frequency of the transmitted signal of varying strength. This causes the simultaneously radiated signals of various frequencies to arrive at the receiver at different times. If the electrical conductivity of the ionosphere is known, the distance of the transmitter from the receiver can be determined from this time shift. The current characteristics for the ionosphere can be determined by ground station, or the time signal transmitter itself continually measures the ionosphere, by analysing the echo of a test signal.
In a further advantageous design of the invention, the earth""s surface is subdivided into numbered zones for saving in memory and computer requirements inside the receiver. The transmitter then transmits a number of the current zone and the previously mentioned supplementary information in addition to the time signal. These data are stored in the receiver. The transmitter therefore can also predict orbit corrections and time change-overs and communicate these to the receiver. By the division of the earth into suitable zones, which do not have to be identical to the international time/zones, the receiver is therefore capable of calculating the actual time in which the receiver is situated, by simple offset- addition or subtraction of the transmitted time information.
The transmitter transmits the actual times and the supplementary information continuously and in an iterative manner. So that the receiver does not have to wait the full period for an already started data packet before the transmission of a complete packet can be started, easily recognised synchronisation signals are embedded in the data stream, so that the analysis can be started in the middle of a packet as well. This minimises the time for which the receiver has to be activated and therefore decreases the electrical current consumption of the clock.
In accordance with international regulations, transmitters on satellites or space stations are not permitted to exceed a certain transmitter power (power flux density), so that other systems are not interfered with. In order to meet this boundary condition, the so-called spread spectrum technique is used in the process according to the invention, actually so that separate encoding and modulation can be carried out. The transmitter signal is then shifted periodically by a given frequency shift in the transmitter frequency (sweeping). This sweeping and all other changes in the transmitter signal occur synchronously and phase-locked to the time standards on board the time signal transmitter, so that the received time can be determined from the instantaneous sweep frequency and the sweep phase position with a resolution into the microseconds range.
For adjustment of the time on board the time signal transmitter, on the one hand control signals from a ground or control station are used, on the other hand the time signal transmitter itself can decode the time signals of national time transmitters during overflight in order to synchronise itself with them.